The present invention relates to an Al—Zn—Mg high-strength aluminum alloy extruded product suitably applied to an impact absorbing structural member such as a bumper reinforcement, a crash box, and a door beam member for vehicles, and a method of manufacturing the same.
An Al—Zn—Mg alloy is known as a high-strength aluminum alloy with excellent extrudability.
A high-strength aluminum alloy extruded product which has been mass-produced and put to practical use has a 0.2% proof stress of about 300 MPa. When further increasing the strength in order to reduce the weight of automobiles, not only extrudability deteriorates, but also breakage tends to occur when impact is applied due to a decrease in toughness. Moreover, since stress corrosion cracking resistance decreases, such an extruded product cannot be applied to an impact absorbing structural member for vehicles.
When increasing the Mg content and the Zn content in order to increase the strength, quench sensitivity after extrusion is increased. This makes it necessary to perform T6 treatment in which the extruded product is subjected to solution treatment and rapidly cooled, thereby resulting in poor productivity.
JP-A-9-310141 discloses an Al—Zn—Mg aluminum alloy extruded product which is a structural material high-strength extruded product designed to provide a specific melt start temperature and subjected to press edge quenching.
The aluminum alloy extruded product disclosed in JP-A-9-310141 aims at ensuring high strength and extrusion productivity. On the other hand, JP-A-9-310141 does not examine the toughness of the aluminum alloy extruded product. The relational expression between the alloy components and the melt start temperature focuses on defects on the surface of the extruded product. Therefore, it is difficult to ensure high toughness.
JP-A-2002-327229 discloses an aluminum alloy extruded product with excellent crushing properties which is suitable for a bumper reinforcement material and the like.
In JP-A-2002-327229, the cooling rate during press quenching is 300° C./min, which is significantly higher than the cooling rate employed for air cooling using a fan.
Since it is difficult to uniformly quench such an aluminum alloy with high quench sensitivity during press quenching, a difference in the degree of cooling occurs between the portion of the material to which cooling air discharged at high speed is directly applied and the remaining portion, whereby distortion in shape such as warping occurs in the extruded product.
In particular, when the extruded product has a hollow cross section, the above distortion in shape such as warping occurs to a large extent due to the heat insulating effect of air in the hollow portion, whereby the product value may not be ensured.